The present invention is concerned with a flexible laminate consisting of an intractable, i.e., no longer moldable, layer of a fully aromatic polyimide and a substrate and a method for forming a polyimide layer directly on a substrate, e.g., copper foil or wire, to form a laminate in which the layers are tightly adhered without an adhesive layer intermediate the polyimide layer and the supporting substrate layer. The polyimides of the present invention are obtained from polyamide acids by an extrusion and curing process which takes place in situ by a process not heretofore known.
The preparation of polyamidocarboxylic acids, commonly referred to as polyamide acids or polyamic acids, which are the precursors or intermediate compounds in the preparation of polyimides, is well known and reference may be made to, e.g. Meyer et al. U.S. Pat. No. 3,981,847 and Sroog et al., J. POLY. SCI., Part A, Volume III, pages 1373-1390 (1965), among others. As is well known, the polyamic acids may be cured to extremely heat-resistant, highly insulating polyimides by a cyclization reaction under the influence of heat or dehydrating agents. Because of the intractability of polyimides with high thermal stability, they must be formed into the desired shape of the end product in the form of the polyamic acid and then or subsequently subjected to cyclization reaction conditions.
The preparation of the polyamic acids is likewise well known, e.g., by the reaction of a tetracarboxylic dianhydride with a primary diamine at temperatures below about 80.degree. C. in an anhydrous, polar, aprotic organic solvent such as those disclosed in the aforementioned U.S. Pat. No. 3,981,847. Also disclosed therein are many of the aromatic dicarboxylic acids, dicarboxylic dianhydrides and aromatic diamines useful in the present invention, which are incorporated herein by reference.
Edwards U.S. Pat. Nos. 3,179,614 and 3,179,634 describe, respectively, methods of preparation of polyamic acids and conversions thereof to the polyimides. These prior patents disclose several methods of coating substrates with polyamic acids and subsequently curing them to polyimide films, for example, by spray gun or dipping techniques followed by air drying at room temperature for several days and then subsequently curing for 30 minutes at 300.degree. C. By these methods, films of up to about 25 .mu.m thickness were prepared, but films or coatings having greater thicknesses required multiple coating and conversion cycles. The drawback observed with multiple coatings onto planar substrates (as opposed to wire) is that adhesion between layers of polyimide coats is extremely low and, therefore, the layers tend to delaminate readily. Polyimide films were also made by casting polyamic acid onto glass plates, drying under vacuum at 50.degree.-80.degree. C. and converting the polyamic acid films to the corresponding polyimides by heating to 300.degree. C. for 30 minutes. The aromatic polyimides disclosed therein are all useful in said invention and said patents are incorporated herein by reference.
European Pat. No. 36,330 describes a process for continuously producing an aromatic polymide film that is soluble in a phenolic-type solvent, e.g., phenol itself or monohalogenated phenol or cresol. On the other hand, the present invention relates to a continuous process for making and shaping a phenol-insoluble, intractable polyimide. While the patent sets forth the desirability of providing a continuous process for the production of polyimides and the extrusion onto a substrate, the disclosed process does not relate to the intractable polyimides of the present invention.
Published European Application No. 48,221 purports to relate to flexible foil substrates provided with a film of polyamide, polyamide-imide or polyimide adhered thereto without an intermediate adhesive layer. However, the only data on actual results relates to the preparation of aromatic polyamides which are soluble and coated in their final, polymerized, form from a solution. No further curing after the film is coated is required. The adhesion of the polyamide film to the substrate is said to be excellent. However, this teaching has no application to polyimides of the present invention, which are insoluble; the insoluble polyimides must be coated on the substrate in the form of their soluble precursor and polymerized in situ.
As to the insoluble polyimides, no specific teaching of conditions required to cure polyamic acids to polyimides in situ is found in European Application No. 48,221, leaving one to consult other references for such conditions. And, of course, the adhesion of the films will be no greater than that obtained by the known prior art, since there is no discussion or disclosure of any modification of conventional processes to increase the adhesion of the polyimides mentioned. One can refer, e.g. to the '614 patent, supra, and, in particular, Example 26, which only states that "good adhesion" is observed on copper, aluminum, steel and glass. However, it has been found that peel strengths of less than 1.7 N/cm are realized.
Notwithstanding the statements just mentioned in European Patent Application No. 48,221 heretofore, it has not been possible to obtain a polyimide (PI) layer of sufficient thickness, i.e., greater than about 25 .mu.m, by a continuous extrusion process permitting a direct application onto a final substrate, e.g., copper foil to obtain a laminate having the required peel strength and necessary electrical properties, e.g., dielectric dissipation factor, dielectric strength, volume resistivity and insulation resistance sufficient to satisfy commercial requirements for flexible printed circuits utilizing such laminates. With the intractable types of polyimides of the present invention, it has been a commercial practice to form unsupported films of the polyamic acid precursor, cure the film and subsequently laminate the polyimide film to a copper substrate with an intermediate adhesive layer, or coat the unsupported polyimide film with a layer of copper, e.g., by vapor deposition. This process, of course, requires an extra step plus the adhesive material used in the laminating process. Moreover, the laminate suffers the technical drawback that available adhesives will not withstand the same high temperatures that the polyimide film can and therefore applications requiring such higher temperatures have been precluded, where, for example, soldering or welding of connections is required. At temperatures reached in the laminate during the soldering, the adhesive softens or melts and the polyimide film "swims" or "floats" in the melted adhesive on the copper surface.
A more desirable process, which would avoid the aforementioned difficulties with adhesives, would be one in which a polyimide solution was extruded directly onto the ultimate supporting sheet after which the solvent thereafter would be removed so as to develop a strong bond between the polyimide film and the substrate. However, the polyimides of the present invention are not soluble in ordinary solvents capable of carrying out the polymerization of polyamic acid. Therefore, the polyimides of the present invention cannot be extruded directly onto the substrate in the same way as the phenol-soluble polyimides of the European Pat. No. 36,330, mentioned above. On the other hand, the polyimides insoluble in phenolic solvents are far superior to the soluble ones as regards theremal stability.
Polyamic acid (PAC) precursors of the aromatic polyimides are soluble, but in previous attempts to coat the polyamic acid onto the substrate and cure the polyamic acid to a polyimide, blisters and bubbles formed, due to a combination of rapid volatilization of the solvent and evolution of water formed by the imidization reaction in the interior of the coating and the formation of a skin of polyimide trapping solvent and water as voids within the film matrix. The discontinuities thereby created in the polyimide layer disrupt the electrical properties required for printed circuit applications of the polyimide laminates, lower the mechanical properties, e.g., tensile strength, elongation to break, etc. of the film and also reduce the strength of the bond between the polyimide film and substrate. A surface layer of polyimide or skin may be formed due to premature curing at the surface layer of PAC and can prevent the release of volatiles (solvent or moisture) by diffusion to the surface, leading to agglomeration of volatile molecules and the formation of voids within the polyimide layer. We have found we can prevent this "skin effect" by programming the temperature of cure so that no PI skin is formed until substantially all of the solvent and volatile products have been diffused to the free surface of the PI layer and released from the surface of the film. Trying to cure thick films prior to substantially complete removal of the solvent leads to the formation of brittle, discontinuous low molecular-weight polyimide film.
U.S. Pat. No. 3,428,602 to Haller addresses the problem of blowing and blistering which is encountered in casting thicker sections of polyimide materials cast as films onto a polytetrafluorloethylene carrier film. Haller suggests that the solvent must be removed from the polyamic acid solution while maintaining the temperature below the heat curing temperature of the polyamide acid because simultaneous removal of the solvent and conversion of the acid to the polyimide causes the blowing and blistering. Moreover, Haller found that after the solvent is reduced to about 50%, merely continuing the heat drying process is ineffectual in lowering the solvent content further. In accordance with the teaching of Haller, after a low solids polyamide acid solution of 12-15% (by volume) is cast into a thin film and heat dried at temperatures below the heat curing temperature of the polyamide acid to lower the solvent content to about 50%, the concentrated polyamide acid solution is then subjected to a shearing operation, for example, in a rubber mill, with further heat drying at temperatures from about 65.degree. C. up to about 149.degree. C. to concentrate the polyamide acid to 75%. The concentrated polyamide acid is then shaped, e.g., by passing it through nip rolls to form a thick sheet which is heated in a curing oven at temperatures ranging from about 149.degree. C. to about 371.degree. C. Haller found it necessary to subject the polyamide acid to the shearing action of a rubber mill to attain sufficient surface exposure to provide the additional drying of the acid solution prior to curing. Furthermore, Haller relates to the formation of free film with greater than 250 .mu.m thickness, but does not relate to the direct extrusion onto a substrate which will adhere thereto with the necessary strength for finished products without the need for an adhesive layer.
German Pat. No. 1,202,981 granted Oct. 14, 1965 discloses a method for preparing shaped polyimide articles by gradually raising the temperature during conversion of the polyamide acid into the polyimide. For example, in Example 16 a pigmented polyamide acid is coated onto a copper substrate and converted by heating into an insoluble polyimide by introducing the film into an oven at 100.degree. C. and raising the temperature to gradually over 35 minutes to 370.degree. C. The films were said to exhibit good adhesion properties, however, the peel strength in those films has been found to be less than 1.7 N/cm. Also, a bubble-free film having a thickness of greater than about 10.mu. cannot be obtained unless the temperature is increased in a certain manner as discovered by Applicants herein and is carefully controlled in the respective temperature zones. For example, as seen in Example 17 of the German patent, ten separate layers are required to obtain a thickness of the coating of 0.023 mm or 0.0023 mm per layer on an AWG #25 wire.
It is an object of the invention to produce a flexible, polyimide laminate without using an adhesive layer, but one which will adhere to the substrate layer with peel strength equivalent to adhesive-laminates and at a cost far below that of conventional adhesive-joined laminates.
Another object of the invention is to produce a polyimide film laminated to a metallic substrate, such as copper, steel, aluminum, zinc, etc., without an adhesive, by directly extruding a polyamic acid onto a copper sheet or foil or the like and curing the laminate thus formed, in situ; the laminate is smooth and free of defects caused by blistering and bubbles probably due, in prior attempts, to a too rapid volatization of either solvent from the free surface of the polyamic acid coating during the curing process or water vapor produced by the imidization reaction or both.
Another object is to achieve, with a single, direct extrusion process, a PI-Cu laminate with a polyimide layer at least 10 .mu.m (0.4 mils) thick, having a peel strength of at least 4 N/cm. (N=Newton), a dissipation factor of 1.5.times.10.sup.-3 to 5.times.10.sup.-3 at 1 Khz and a dielectric strength of at least about 2 KV/mil.
A further object is to achieve a polyimide laminate with high peel strengths able to withstand high temperatures used in processing such laminates into useful products, such as temperatures reached in soldering connectors to printed circuit boards made from said laminates, without the necessity for pre-drying to remove water from the polyimide layer or trapped in the adhesive layer.
A further object is to produce a wire coated with a single polyimide layer greater than about 64 .mu.m (21/2 mils) and a process for coating in a single pass and curing in situ.